Development of permporometry for analysis of MFI membranes

Abstract: Zeolite membranes exhibiting high flux and high selectivity are of major interest for potential future applications. In order to achieve high flux and high selectivity, the zeolite film must be thin (< 1 µm) and free from flow-through defects. The development of thin defect free zeolite membranes requires powerful tools for characterization of flow-through defects in the membranes. Permporometry is one of the most straightforward and powerful techniques for characterization of flow-through pores in ceramic membranes. In permporometry, the flow of a non-condensable gas, e.g., helium, through the membrane is monitored as a function of the activity of a strongly adsorbing compound, e.g., hydrocarbon. In the present work, MFI membranes prepared by a seeding method were characterized by permporometry using helium as the non-condensable gas and n-hexane or benzene as the adsorbing compound. In order to appreciate permporometry data, the membranes were also characterized by scanning electron microscopy (SEM), single gas permeation and separation experiments. The permporometry data were then compared to the SEM morphology of the membranes, permeances of different probe molecules and membrane separation performance. In order to determine the conditions of the permporometry experiment leading to blocking of zeolite pores, a model describing helium transport in the zeolite pores in the presence of n-hexane or benzene was developed. The model is based on percolation theory and knowledge of the adsorption isotherms and adsorption sites for n-hexane and benzene in the zeolite pores. Parameters needed in the model were estimated by Density Functional Theory (DFT) using a Local-Density Approximation (LDA), the most sophisticated theory yet applied to this system. Based on the permporometry data, it was demonstrated that the model could adequately describe helium transport in zeolite pores in the presence of the hydrocarbons. The sensitivity of the permporometry technique towards the defect size has been improved considerably. It was revealed that high quality MFI membranes prepared in the present work contained mainly micropore defects which are most like the defects in the zeolite crystal lattice (intracrystalline defects). The work has shown how permporometry data could be used to estimate the area distribution of the flow-through defects in the membranes. The results on the defect distribution were corroborated by the SEM observations and the separation experiments. The width of cracks, including support cracks, and open grain boundaries observed by SEM was in excellent agreement with the defect width estimated from permporometry data. A straightforward correlation was observed between separation data and permporometry data, i.e. membranes of higher quality according to permporometry analysis exhibited greater separation performance. Also, the permeance of molecules diffusing through defects in the membrane in the separation experiment was found to scale with the permeance of helium through the defects measured in thepermporometry experiment. In addition, this work showed that single gas permeance ratios could not detect slight variations in the membrane quality. For membranes with similar however slightly different amount of defects, the ratios are mainly affected by the membrane thickness and support morphology. To summarise, the present work demonstrates that permporometry data adequately reflect membrane quality and that permporometry is a very powerful technique for MFI membrane characterization.